1. Field of the Invention
The present invention relates to an electric power steering apparatus designed to assist the steering force of a steering member by an electric motor.
2. Description of Related Art
In a conventional electric power steering apparatus, when a switching element constituting a driving circuit of an electric motor is damaged and short-circuits, a relay contact of a power supply system is turned off to cut off power.
However, in the case of a brushless motor, for example, even when the circuit is disconnected and power is cut off, a circuit in which an induced current by the brushless motor flows is formed in at least one of U-, V-, and W-phase circuits of the driving circuit. Therefore, when a steering wheel is operated without activating the electric power steering apparatus, the brushless motor functions as a generator and generates an induced current. Then, there are problems that the steering wheel may become extremely heavy and a variation (torque ripple) may occur in the weight of the steering wheel.
In order to solve these problems, conventionally, the following measures have been taken: a relay contact is connected to a path through which the induced current flows, and the relay contact is turned off at occurring of a short-circuit failure; a joint device such as a clutch is interposed between the motor and the steering system, and the joint device is disconnected at occurring of a short-circuit failure so as to prevent the motor from being rotated by the steering wheel and generating power; further a switching device is disposed in series to the motor, and the switching device is opened when abnormality occurs so as to cut off the path through which the induced current flows.
However, all of these measures suffer from the problems of complicated structures, an increase in the number of parts, and a rise in the cost of parts.
Note that, as a technique for disconnecting the circuit, Japanese Patent Application Laid-Open No.11-250790 (1999) discloses a forced fusing fuse for forcibly fusing a fuse, and a current cutting device. Moreover, Japanese Patent No. 2662315 discloses an overheat preventing device for a heater, for preventing overheating of a heating unit by detecting an abnormal condition in which a temperature fuse portion of an electric fan heater, an electric carpet, or the like does not work properly. Further, Japanese Patent Application Laid-Open No. 2001-43783 discloses protection devices, such as a current fuse, a temperature fuse and a fuse with a resistor, which eliminate the necessity of heating a sealing resin at sealing of an insulation case with the sealing resin, and thereby prevent fusion of a low-melt point alloy or a soluble alloy at sealing and prevent error operation in an actual operation.